In large modern buildings such as office buildings, multiunit residences, etc., there are often large networks of floors, hallways, or tunnels. The hallways or tunnels may run in long, straight paths or may abruptly turn at sharp angles. Buildings are constructed using structural steel supports or metallic wall studs. Also, aircraft and ships often have confined passageways. Such buildings, hallways, or tunnels provide a very challenging environment for wireless communications. Yet, there is a need for practical wireless communication systems which provide two-way communication between a base station, which might be in a particular office cf the building, and portable communication equipment carried by maintenance or security personnel. Preferably, the personnel should be able to use standard portable radio equipment or other standard equipment for wireless electromagnetic (EM) communication.
In order for a wireless communication system to work effectively inside enclosed areas such as hallways or tunnels, two requirements must be met. First, an EM field carrying signals transmitted from the base station to the personnel must have substantially equal field strength for all regions in the areas. Second, the system must be approximately equally sensitive to signals transmitted by the personnel using their portable equipment regardless of the location of the personnel within the enclosed area. These objectives have proven very difficult to realize in practice.
The structural members and other reflective surfaces which were used in construction of the enclosed areas provide a bewildering maze of reflective surfaces for EM radiation. Accordingly, enclosed areas experience multipath distortion, including deep nulls due to factors such as phase cancellation. The distortion and nulls are distributed throughout the enclosed areas. As a practical matter, the complexity of the structure of enclosed areas makes it impossible to predict by analysis where the distortion and nulls will be located. A mere few inches may separate an area of strong, clear reception from a null area in which reception is nearly inaudible. Accordingly, it is impracticable to design a system in which the distortion and nulls may be located in out-of-the-way places.
Conventional communications systems have been used including multiple conventional antennas, multiple feedlines, and power splitters in order to propagate signals around sharp bends or bulkheads and reduce or eliminate nulls. Such systems, however, are disadvantageously complex and expensive, particularly because transmitters and receivers used at the base station and used by the remote personnel must be able to d al with a large standard deviation of signal strength, i.e., from deep nulls up to areas of strong reception.
A simpler and less expensive communication system has been used employing slotted coaxial cable antennas running along hallways or tunnels. A slotted coaxial cable antenna is mechanically simpler and cheaper than other conventional systems, but is still subject to multipath interference effects, including nulls. In order to make the system usable in spite of the distortion and nulls, high power transmitters have been used, so that even nulls are boosted enough that their signals are audible. However, this brings about the additional disadvantage of undesirably high power consumption. The additional power, or "system use factor", may require on the order of 20 dB of additional power for satisfactory signal strength.
A further technique has been used for designing systems for overcoming the problem of distortion and nulls. Two or more distributed antennas having different characteristics are laid throughout the enclosed area. Due to the different characteristics, if a given position in the enclosed area has a null for one antenna, the likelihood is small that another antenna will also have a null there. Communication equipment may be used which alternates on a time division basis between the signals of the antennas, or which has intelligent circuitry for selecting the strongest signal. Thus, the communication problems caused by distortion and nulls are largely eliminated. However, considerable additional cost is still incurred due to the redundant antennas and the more specialized and complex communication equipment.
Therefore, there remains a need for a simple and inexpensive, yet effective, system for wireless communication in enclosed areas.